A switch (sometimes referred to as a gateway) is used to supply Public Switched Telephone Network (PSTN) connectivity through a plurality of interfaces, the interfaces including T1, T3, E1, E3, DS1, DS3, analog ports and the like. Within the gateway, the physical network interfaces are terminated on a Rear Transmission Module (RTM) while the processing function is implemented by Trunk Processing Modules (TPMs).
Redundancy schemes may be implemented for certain applications involving a media gateway. “N+1” means that there is one, and only one, redundant element for N active elements. If one element fails then it is replaced by the spare (“+1”) element. If two active elements fail, there is a degradation of service because only one of these elements has been replaced. “N:1” means that there are N active elements, any of which can communicate to the “1” at any time. In the latter case, an N:1 system can be made to look like an N+1 system, but an N+1 system can never function as a N:1 system.
Current designs provide a fixed 1 to 1 mapping between the Line Interface Unit (LIU)/Framer hardware inserting and extracting signaling and control information on the service connection and the local CPU driving that hardware. This design requires relays and/or analog switches to switch between redundant line cards and/or logic in redundant LIU/Framer devices. Current solutions use a relay (or equivalent) switching mechanism to protect the layer 1 transmission connection between two T-Carrier switches.